1. Field of the Invention
This invention relates to a compact bias structure to efficiently package an array of bubble domain devices within a uniform rotating magnetic field.
2. Description of the Prior Art
Prior art packaging of bubble domain devices is limited by the means in which a required in-plane rotating magnetic field is generated. For example, a conventional wire wound coil configuration to generate a magnetic field requires that a bubble device be enclosed in a respective closely wound coil structure. This type of configuration employs separate packages. Consequently, a relatively large number of electrical components and interconnections therebetween are employed when an array (e.g., such as that comprising a magnetic bubble memory) of such bubble devices is to be utilized. Thus, the size and expense of fabricating a large array is undesirably increased. What is more, the confined structure of a bubble device and the closely wound coil prevents easy access thereto for repair or replacement. Moreover, the close proximity of a coil winding to a bubble device increases the adverse effects of inductance and associated heat generation.
One example of a known open coil structure is that which utilizes a flat Helmholtz type coil having a ferrite core inserted into the coil. For flux closure, a ferrite ring encircles the assembly. However, although the use of ferrite pole pieces can improve the magnetic field uniformity, they also introduce distortions as a result of the coupling of orthogonally disposed pole pieces. The sensitivity of this arrangement is also low. This is due to the large leakage path between pole pieces and to the large demagnetization factor of the individual pole pieces. The overall efficiency of this network can be improved by matrixing the coil windings so that the field from both ends of a respective pole piece can be utilized. This results in a large field network which requires that all of the coil windings in one plane be energized simultaneously and, thus, by a large driving power. Moreover, all of the devices and coils must be aligned in one plane which introduces packaging problems. What is more, ferrite poles will reduce the efficiency of a bias structure and also interfere with a rotating field in other device planes placed above or below.
Another example of a known open coil structure is that which utilizes a flat faced coil. Instead of using a closed winding, the return path of the conductor current is spread into a plane formed from a flat spiral coil. Two sides of this coil are straight with parallel wires. When two of the coils are placed face to face, the magnetic field in the area between the straight sections of the coil is identical to that inside a flat solenoid coil. By placing two sets of these spiral coils perpendicular with respect to each other, a rotating field can be achieved. However, the coil area cannot be fully used because of the circular part of the spiral winding and the center open space in each of the flat faced coils which is required to minimize the interference between opposite current paths. Thus, only a limited area of the face to face flat coils is usable, and each of the usable areas has a different rotating field phasing and direction. A pair of bubble memory chips with two different sense and rotation (i.e., clockwise and counterclockwise) must be used. Moreover, this arrangement does not permit stack packaging without additional decoupling provided by ferrite plates between the face to face coil planes.